The
nematodes or roundworms are one of the most common phyla of animals,
with more than 20,000 described species. They inhabit freshwater,
marine, and terrestrial environments, where they often outnumber
other animals in both individual and species counts, and are found
in locations as diverse as Antarctica and oceanic trenches. Further,
there are a great many parasitic forms, including pathogens in
most plants and animals, humans included. Only Phylum Arthropoda
exceeds Nematoda in diversity. Nematodes are unsegmented, bilaterally
symmetric, have triploblastic protostomes, and possess a complete
digestive system. Nematodes are believed to be related to the
arthropods and priapulids and grouped with them in the Ecdysozoa
(the molting animals), the evolutionary is unresolved. Because
most living forms are microscopic, the discovery of their ancient
ancestors as fossils is unlikely. However, one species of extant
parasitic nematode can reach 13 meters in length. lacking notable
mineralized body parts, chances for fossilization of soft tissues
is rare, and require special circumstances as existed for the
specimens from the Cambrian Chengjiang fossils shown below, which
also suggest their evolutionary appearance in the Precambrian.
Nematode fossils have been found in amber.